1. Field of the Invention
The present invention relates to a surface acoustic wave device, and particularly to a surface acoustic wave device for use in a comparatively high frequency area in a radio communication system or the like.
2. Description of the Related Art
In a manufacturing process of such a surface acoustic wave device, there is a case where interdigital transducers may be electrostatically broken by heating applied when a piezoelectric substrate is bonded to a package, or by temperature rising generated when a cap is welded, or the like. This electrostatic break is caused by pyro-electricity of the piezoelectric substrate. To prevent this, there is such a proposal as disclosed, for example, in JP-A-6-224682.
In this proposal, electrostatically breaking sacrificial electrodes are provided at part of input and output electrodes, so that these sacrificial electrodes are electrostatically broken actively to thereby prevent excitation portions of the input and output electrodes from being eliminated.
It is also well known that to cover a surface of a piezoelectric substrate with a thin film electrode as wide as possible is effective, because charges due to spontaneous polarization caused by pyro-electricity are accumulated in a free surface where the substrate surface is not short-circuited.
However, since the electrostatically breaking sacrificial electrodes according to the above-mentioned publication are provided at part of input and output electrodes, there arises a problem that the frequency characteristic deteriorates because of electrostatic capacity of the electrostatically breaking sacrificial electrodes, particularly when they are used with a high frequency.
In addition, there arises a problem that, when the electrostatically breaking sacrificial electrodes are blown out by static electricity, melted electrode material deposits between the electrostatically breaking sacrificial electrodes to thereby reduce the insulation resistance at that place. Further, though rarely, the electrostatically breaking sacrificial electrodes are short-circuited by the melted electrode material. Since either one of the input and output electrodes is short-circuited at that time, there arises an extremely serious fault that high-frequency signals cannot pass there. In addition, even when the surface of the piezoelectric substrate is covered with a thin film electrode as wide as possible, there may be a case where electrostatic breaking of the interdigital transducers cannot be prevented.
FIG. 5 shows a surface acoustic wave device in the background art schematically. Input and output interdigital transducers 2 are formed on a piezoelectric substrate 1. These electrodes are connected to an input wire bonding pad 3 and an output wire bonding pad 4 through common electrodes 6. In addition, a ground bonding pad 5 is attached to each of the interdigital transducers 2. Thin film electrodes 7i and 7j for short-circuiting charges caused by spontaneous polarization are provided between these electrodes and the chip edge surface.
The thin film electrodes 7i and 7j are not formed to the edge surface of the piezoelectric substrate 1, but a dicing margin portion 9 is provided between the edge surface of the piezoelectric substrate 1 and the edges of the thin film electrodes 7i and 7j. This is because the interdigital transducers 2 are prevented from short-circuiting because of a broken piece produced by dicing a wafer to cut the piezoelectric substrate 1 out.
Although the surface acoustic wave device configured thus has an effect that the accumulation of electric charges due to spontaneous polarization can be restrained by the thin film electrodes 7i and 7j, the interdigital transducers 2 are still electrostatically broken. This is because the electric charges of spontaneous polarization generated in the dicing margin portion 9 which is the widest free surface in the piezoelectric substrate 1 is discharged into the interdigital transducers 2 through the thin film electrodes 7i and 7j.
To prevent this, it can be considered that spaces S2 and S3 are provided between the thin film electrodes 7i and 7j and the interdigital transducers 2 so as to prevent them from connecting to each other electrically, as shown in FIG. 6. High static electricity reaching the value of several kV may break the insulation of the spaces S2 and S3 to bring electrostatic breaking. Although the insulation is not broken if the spaces S2 and S3 are extended sufficiently, the free surface is widened by the spaces S2 and S3 so that the initial object cannot be attained.